William was last seen wearing a Spiderman costume in his grandmother’s backyard in Kendall, near Port Macquarie on September 12, 2014.

The reward is the largest ever in NSW’s history, and unlike other rewards, this is for the location of the child and is not contingent on a conviction, reports SMH.

"It's unacceptable that two years down the track we haven't found out what's happened to William," said Detective Chief Inspector Gary Jubelin who assured anyone who has information regarding William’s location they would not be in trouble for not coming forward sooner.

"I'm confident in saying it's only a matter of time before we find out what's happened to William and if we come to you, the offer of the reward is off the table."